1. Field of the Invention
This invention relates to a toner for developing electrostatic images, for example, in electrophotography and electrostatic printing.
2. Description of the Prior Art
Processes for developing electrostatic images can be generally divided into a liquid type process where a liquid developer composed of a fine toner dispersed in an electrically insulating layer and a dry type process where a fine toner composed of a natural or synthetic resin containing a dispersed colorant is used alone or in combination with a solid carrier.
Heretofore there are known various methods for visualizing electrostatic latent images with a toner such as a magneto-brush developing method is U.S. Pat. No. 2,874,063, a cascade developing method in U.S. Pat. No. 2,618,552, a powder cloud developing method in U.S. Pat. No. 2,221,776 and the like. As a developing toner used in these developing methods, there are used finely divided materials composed of dyestuffs or pigments dispersed in binders, and further, developing toners containing a third material in addition to the colorant and the binder such as those disclosed in Japanese Patent Publication Nos. 11096/1963, 10866/1965, 6398/1969 and the like.
Since a desirable charge on the toner particle can not be obtained by a binder resin alone, there is often used a dyestuff, pigment or tribo controlling agent for imparting a desirable charge. At present, nigrosine series dyestuffs are used for imparting positive charge, and metal-containing dyestuffs as disclosed in Japanese Patent Publication No. 26478/1970 are used for imparting negative charge. These dyestuffs are much better than conventional charge controlling agents with respect to imparting a charge to toner. These dyestuffs are, however, disadvantageously colored materials.
In general, toners used for multicolor electrophotography must have particular limited colors such as yellow, magenta and cyan. It is clear that bluish purple nigrosine series dyestuffs or metal-containing dyestuffs can not be used as a charge control agent for the toners in multicolor electrophotography. It is, therefore, necessary to use a charge controlling agent which is colorless or substantially colorless such as light color.
Another disadvantage of conventional toner having a binder resin containing a dyestuff or pigment is that the toner shows good developing characteristics at the beginning but rapidly loses such good characteristics.
The reason is that such dyestuff or pigment is not compatible with the binder resin or is of low wettability with respect to the binder resin. In other words, according to conventional methods of producing toners, a binder resin, a colorant, a charge controlling dyestuff or pigment and others are mixed and melted and finely divided to powders of about 1-50 microns in size. In this procedure, there are formed powders which components are not uniform and fine particles composed of the charge controlling dyestuff or pigment only. These fine particles composed of only the charge controlling dyestuff or pigment adversely affect the life of the developer.
Such undesirable charge controlling fine particles are formed in a step where a developer is used in a copier. During the developing step, the developer is continuously stirred and rubbed against an electrostatic latent image retaining surface such as a surface of a photosensitive member and therefore, bare charge controlling fine particles on the surface of a toner particle which is hardly wetted by the binder resin fall from the toner particle. The charge controlling fine particles are apt to generate intensely triboelectric charge with such a carrier as iron and therefore, strongly and electrically attach to such a carrier. As the result, amount of triboelectric charge between the carrier and the toner decreases or the polarity becomes unstable, or electric resistance of the carrier such as iron powder increases, and thereby, the density of images decreases and fog is formed and further, edge effects appear.